Wireless broadband access communication systems as for example WIMAX systems, foresee the support of multicast/broadcast services as defined in IEEE 802.16e-2005 standard. More precisely, some globally defined service flows may carry broadcast or multicast information that should be delivered to a plurality of end user stations. Since a multicast or broadcast transport connection is associated with a service flow, it is associated with the QoS and traffic parameters for that service flow.
Two types of access to multicast and broadcast services (MBS) may be supported: single base station (Single_BS) access and multi-base station (Mutiple_BS) access. Single-BS access is implemented over multicast and broadcast transport connections within one base station, while multi-BS access is implemented by transmitting data from Service Flow(s) over multiple base stations. A user terminal may support both Single-BS and Multi-BS access. Initiation of MBS with respect to specific user terminals is always performed in registered state by creation of multicast connection carrying MBS data. During such initiation the user terminal learns the Service Flow ID that identifies the service. For multi-BS-MBS, each base station capable of providing MBS belongs to a certain MBS Zone, which is a set of base stations where the same Connection Identifier (CID) is used for transmitting content of certain Service Flow(s). MBS Zone is identified by a unique MBS_ZONE identifier. In multi-BS-MBS all user terminals successfully registered can receive on the cell the MAC PDUs of the multicast and broadcast content that multiple base stations transmit anywhere under the given time period. It requires the multiple base station participating in same Multi-BS-MBS service to be synchronized in the transmissions of common multicast/broadcast data. To ensure proper multicast operation, the CID used for a multi-BS-MBS connection shall be the same for all base stations and user terminals on the same channel that participate in the connection.
Multicast service synchronized across multiple base station enables a user terminal to receive the multicast or broadcast transmission from multiple base station, and thereby improve the reliability of reception using macro diversity.
For this purpose and as already mentioned, MBS transmission in a group of base station should be synchronized. In such case, each base station shall transmit the same PDUs, using the same transmission mechanism (symbol, subchannel, modulation, and etc.) at the same time. The way that multiple base stations accomplish the synchronized transmission (which implies performing functions like classification, fragmentation, scheduling at a centralized point called the MBS Server) is of primary importance for a properly working system.
Indeed, since the transport of broadcast multicast data is IP based, the same packets may be received at very different time at the different base stations from the network side. Consequently, the base station may include the same packet in two different PDUs, generating completely different MBS portions at different base stations and rendering the use of macro diversity impossible at receiver side.
Consequently, a particular object of the present invention is to provide a method for appropriate synchronization for multicast and broadcast services in wireless broadband access networks.
Another object of the present invention is to provide a multicast and broadcast server in accordance with the method.
Another object of the invention is to provide a corresponding base station in accordance with the method.